Ofdm/ofdma frame structure for communication systems

ABSTRACT

An OFDM/OFDMA frame structure technology for communication systems is disclosed. The OFDM/OFDMA frame structure technology comprises a variable length sub-frame structure with efficiently sized cyclic prefixes and efficient transition gap durations operable to effectively utilize OFDM/OFDMA bandwidth. Furthermore, the frame structure provides compatibility with multiple wireless communication systems. An uplink frame structure and a downlink frame structure are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/980,760 filed on Oct. 17, 2007, U.S. Provisional PatentApplication No. 61/032,032 filed on Feb. 27, 2008, and U.S. ProvisionalPatent Application No. 61/020,690 filed Jan. 11, 2008, the contents ofwhich are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to digital communications andmore particularly to Orthogonal Frequency Division Multiplexing (OFDM)and Orthogonal Frequency Division Multiple Access (OFDMA) systems.

BACKGROUND OF THE INVENTION

There is an increasing need for mobile high speed communication systemsto provide a variety of services such as the Internet, television, photosharing, and downloading music files. In order to provide such services,a mobile high speed communication system must be able to overcome avariety of difficult operating conditions caused by the environment.Among these operating conditions are multipath signals, inter-symbolinterference (ISI), and inter-channel interference (ICI). In mobile highspeed communication systems, multipath is interference resulting fromradio signals reaching the receiving antenna by two or more paths.Causes of multipath include atmospheric ducting, ionospheric reflectionand refraction, and reflection from terrestrial objects, such asmountains and buildings. In telecommunications, ISI is a form ofdistortion of a signal in which one symbol interferes with subsequentsymbols. ICI is a form of distortion of a signal caused by transmissionof signals on adjacent channels that may interfere with one another.

FIG. 1 illustrates a mobile radio channel operating environment 100. Themobile radio channel operating environment 100 may include a basestation (BS) 102, a mobile station (MS) 104, various obstacles106/108/110, and a cluster of notional hexagonal cells126/130/132/134/136/138/140 overlaying a geographical area 101. Eachcell 126/130/132/134/136/138/140 may include a base station operating atits allocated bandwidth to provide adequate radio coverage to itsintended users. For example, the base station 102 may operate at anallocated channel transmission bandwidth to provide adequate coverage tothe mobile station 104. The base station 102 and the mobile station 104may communicate via a downlink radio frame 118, and an uplink radioframe 124 respectively. Each radio frame 118/124 may be further dividedinto sub-frames 120/126 which may include data symbols 122/128. In thismobile radio channel operating environment 100, a signal transmittedfrom a base station 102 may suffer from the operating conditionsmentioned above. For example, multipath signal components 112 may occuras a consequence of reflections, scattering, and diffraction of thetransmitted signal by natural and/or man-made objects 106/108/110. Atthe receiver antenna 114, a multitude of signals may arrive from manydifferent directions with different delays, attenuations, and phases.Generally, the time difference between the arrival moment of the firstreceived multipath component 116 (typically the line of sightcomponent), and the last received multipath component (possibly any ofthe multipath signal components 112) is called delay spread. Thecombination of signals with various delays, attenuations, and phases maycreate distortions such as ISI and ICI in the received signal. Thedistortion may complicate reception and conversion of the receivedsignal into useful information. For example, delay spread may cause ISIin the useful information (data symbols) contained in the radio frame124.

Orthogonal Frequency Division Multiplexing (OFDM) is one technique thatis being developed for high speed communications that can mitigate delayspread and many other difficult operating conditions. OFDM divides anallocated radio communication channel into a number of orthogonalsubchannels of equal bandwidth. Each subchannel is modulated by a uniquegroup of subcarrier signals, whose frequencies are equally and minimallyspaced for optimal bandwidth efficiency. The group of subcarrier signalsare chosen to be orthogonal, meaning the inner product of any two of thesubcarriers equals zero. In this manner, the entire bandwidth allocatedto the system is divided into orthogonal subcarriers.

Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-userversion of OFDM. For a communication device such as the base station102, multiple access is accomplished by assigning subsets of orthogonalsub-carriers to individual subscriber devices. A subscriber device maybe a mobile station 104 with which the base station 102 iscommunicating.

An inverse fast Fourier transform (IFFT) is often used to form thesubcarriers, and the number of orthogonal subcarriers determines thefast Fourier transform (FFT) size (N_(FFT)) to be used. An informationsymbol (e.g., data symbol) in the frequency domain of the IFFT istransformed into a time domain modulation of the orthogonal subcarriers.The modulation of the orthogonal subcarriers forms an information symbolin the time domain with a duration T_(u). Duration T_(u) is generallyreferred to as the OFDM useful symbol duration. For the subcarriers toremain orthogonal, the spacing between the orthogonal subcarriers Δf ischosen to be

$\frac{1}{T_{u}},$

and vice versa the OFDM symbol duration T_(u) is

$\frac{1}{\Delta \; f}.$

The number of available orthogonal subcarriers N_(C) (an integer lessthan or equal to N_(FFT)) is the channel transmission bandwidth (BW)divided by the subcarrier spacing

$\frac{B\; W}{\Delta \; f},$

or BW*T_(u).

FIG. 2 illustrates principles of an OFDM/OFDMA multicarrier transmissionwith four subcarriers. The principle of multi-carrier transmission is toconvert a serial high-rate data stream 202 into multiple parallellow-rate sub-streams 204 by a serial-to-parallel converter. Eachparallel sub-stream is modulated on to one of N_(C) orthogonalsub-carriers 206, where N_(C) is an integer that, for example, can begreater than or equal to 128. The N_(C) sub-streams are modulated ontothe N_(C) sub-carriers 206 with a spacing of Δf in order to achieveorthogonality between the signals on the N_(C) sub-carriers 206. Theresulting N_(C) parallel modulated data symbols 210 are referred to asan OFDM symbol. Since the symbol rate on each sub-carrier 206 is muchless than the symbol rate of the initial serial data 202, the OFDMsymbols are less sensitive to timing. Thus, the effects of symboloverlap (i.e., ISI) caused by delay spread decrease for the channel.

FIG. 3 illustrates ISI between OFDM/OFDMA symbols. As shown in FIG. 3,OFDM/OFDMA symbols S1-S3 may be transmitted on the sub-frame 120 of thedownlink radio sub-frame 118 from the base station (BS) 102 to themobile station (MS) 104 (FIG. 1). Multipath components 112 (FIG. 1) maycause a delay spread 302 of the symbols S1-S3. The delay spread maycause the OFDM/OFDMA symbols S1-S3 to overlap each other, such that ISI304 occurs between OFDM/OFDMA symbols S1-S2 and S2-S3. If the ISI islarge enough, the signal reception may be disrupted.

In order to make an OFDM/OFDMA system more robust to multipath signals,an extension is made to the information symbol called a cyclic prefix.The cyclic prefix 402 is generally inserted between adjacent OFDM/OFDMAsymbols as shown in FIG. 4. The cyclic prefix 402 is typicallypre-pended to each OFDM/OFDMA symbol and is used to compensate for thedelay spread introduced by the radio channel as explained below. Thecyclic prefix 402 can also compensate for other sources of delay spreadsuch as that from pulse shaping filters often used in transmitters. Bysignificantly reducing or avoiding the effects of ISI and ICI, thecyclic prefix 402 also helps to maintain orthogonally between theOFDM/OFDMA signals on the sub-carriers 206 (FIG. 2). The cyclic prefix402 has a duration T_(G), which may be added to the useful symbolduration T_(u). Thus, a total OFDM/OFDMA symbol duration T_(SYM) may beT_(u)+T_(G). Although, in this example, a total OFDM/OFDMA symbolduration of T_(SYM)=T_(u)+T_(G) may be employed for transmitting anOFDM/OFDMA symbol, only the useful symbol duration T_(u) (FIG. 2) may beavailable for user's data symbol transmission.

As mentioned above, the cyclic prefix 402 is a cyclic extension of eachOFDM/OFDMA symbol, which is obtained by extending the duration of anOFDM/OFDMA symbol. FIG. 5 shows an exemplary cyclic prefix. In FIG. 5, asinusoidal curve 504 corresponds to an original sinusoid where one cycleof the sinusoid is of duration 3.2 μs (i.e., 64 samples with 20 MHzsampling rate). For this example, the subcarrier frequency is 312.5 KHz.A cyclic prefix 502 of 16 samples (0.8 μs) is pre-appended to theoriginal subcarrier 504 which still has the original sinusoid offrequency 312.5 KHz. The sinusoid is now of duration 4.0 μs, whichallows the receiver to choose one period (3.2 μs) of the subcarrier 504from the bigger window (4.0 μs). In this manner, the cyclic prefix 502acts as a buffer region. The receiver at the mobile station 104 (FIG. 1)may exclude samples from the cyclic prefix 502/402 that are corrupted bythe previous symbol when choosing samples for OFDM/OFDMA symbols (e.g.,S1-S3 (FIG. 3)). The cyclic prefix 502/402 duration should be optimizedto increase bandwidth efficiency (i.e., bit/Hz).

In telecommunications, a frame is a fixed or variable length packet ofdata, which has been encoded by a communications protocol for digitaltransmission. A frame structure is the way a communication channel isdivided into frames (e.g., 118/124 in FIG. 1) or sub-frames (e.g.,120/126 in FIG. 1) for transmission. The frame structure of an OFDM orOFDMA system contributes to determining the performance of acommunication system. In a communications system, the size and timing ofa cyclic prefix in a frame is specified by a frame structure.

In existing OFDM/OFDMA systems, such as Wireless Interoperability forMicrowave Access (WiMAX), the cyclic prefix is configurable, but it isfixed when a system is deployed. This limits configuration of the systemfor efficient bandwidth utilization since the cyclic prefix cannot bereconfigured. Additionally, in existing frame structures, there are nomechanisms to allow a base station to change or reconfigure the cyclicprefix duration for different communication usage scenarios. Forexample, when communication in a channel suffers from severe multipatheffects (i.e., large delay spread), a longer cyclic prefix can be usedto eliminate the ISI and ICI. In less severe channel conditions, withfewer multipath issues, a short cyclic prefix can be used in order toreduce overhead and transmission power. Therefore, there is a need forsystems and methods that provide a frame structure for high performanceOFDM and OFDMA systems that more efficiently use the cyclic prefix.

SUMMARY OF THE INVENTION

An OFDM/OFDMA frame structure technology for communication systems isdisclosed. The OFDM/OFDMA frame structure technology comprises avariable length sub-frame structure with efficiently sized cyclicprefixes, and efficient transition gap durations operable to effectivelyutilize OFDM/OFDMA bandwidth. Furthermore, the frame structure providescompatibility with multiple wireless communication systems. An uplinkframe structure and a downlink frame structure are provided.

A first embodiment comprises an OFDM/OFDMA communication system. TheOFDM/OFDMA communication system comprises a plurality of radio frequency(RF) channels, wherein the RF channels comprise dissimilar bandwidths.The system also comprises a transmitter for providing a plurality ofOFDM subcarriers. The OFDM subcarriers comprise a fixed subcarrierspacing chosen such that the OFDM subcarriers are scalable in number toutilize any of the RF channels. In one embodiment, all RF channelbandwidths in the communication system can be divided evenly by thesubcarrier spacing.

In addition, the system can further comprises a processor coupled to thetransmitter and operable to provide a flexible radio frame structurecomprising a plurality of variable length cyclic prefixes operable forthe RF channels.

A second embodiment comprises a communication system. The communicationsystem comprises at least one base station supporting variable cyclicprefix durations. The variable cyclic prefix durations are chosen basedon a cell coverage area of the at least one base station. The systemalso comprises a processor for providing a flexible radio framestructure utilizing the variable size cyclic prefix durations. Theflexible radio frame structure is used by the at least one base stationfor transmitting data to a mobile station. The processor may also beoperable to calculate a plurality of timing gaps associated with atleast one of the sub-frames, wherein the timing gaps are calculatedbased in part on the variable cyclic prefix durations.

A third embodiment comprises an OFDM/OFDMA radio frame structure forcommunication in an RF channel in a wireless network. The radio framestructure comprises a plurality of OFDM symbols each comprising avariable cyclic prefix duration and at least one OFDM data symbol. Theframe structure also comprises a plurality of variable size sub-framesformed from a subset of the OFDM symbols, and a plurality of radioframes for transmitting a subset of the variable sub-frames through theRF channel. The frame structure further comprises a plurality of timinggaps associated with the radio frames for providing a protection fortiming variations at signal reception. The timing gaps are calculatedbased, at least in part, on the variable cyclic prefix duration.

A fourth embodiment comprises a communication system. The communicationsystem comprises a plurality of RF channels, wherein a subset of the RFchannels have dissimilar channel bandwidths. The system also comprisesan inverse fast Fourier transform (IFFT) module operable fortransforming a plurality of frequency domain data symbols into aplurality of time domain data symbols respectively. The system furthercomprises a cyclic prefix selector operable for selecting a cyclicprefix from a plurality of variable length cyclic prefixes to obtain aselected cyclic prefix. The system also comprises an add cyclic prefixmodule operable for adding the selected cyclic prefix to each of thetime domain data symbols to obtain a plurality of OFDM frames.

The system may also comprise a processor operable for providing aplurality of variable size sub-frames formed from a subset of the OFDMframes. The processor is also operable for providing a plurality ofradio frames for transmitting a subset of the variable size sub-framesthrough at least one of the RF channels. The processor is furtheroperable for calculating a plurality of timing gaps associated with atleast one of the variable size sub-frames for providing a protection fortiming variations at signal reception. The timing gap is calculatedbased, at least in part, on a cyclic prefix duration of the selectedcyclic prefix.

A fifth embodiment comprises a method for communication in acommunication system. The method comprises receiving a time domain datasymbol for transmission on a radio channel, and selecting a cyclicprefix from a plurality of variable length cyclic prefixes to obtain aselected cyclic prefix. The method also comprises adding the selectedcyclic prefix into each of the time domain data symbols to obtain aplurality of OFDM frames.

A sixth embodiment comprises a computer-readable medium for acommunication system. The computer-readable medium comprises programcode for receiving a time domain data symbol for transmission on a radiochannel. The program code also selects a cyclic prefix from a pluralityof variable length cyclic prefixes for the radio channel to obtain aselected cyclic prefix. The program code also adds the selected cyclicprefix into each of the time domain data symbols to obtain a pluralityof OFDM frames.

The computer-readable medium may further comprise program code foradding the OFDM frames to a flexible sub-frame prior to transmitting theOFDM frames on the radio channel. The program code may also provide aplurality of variable size sub-frames formed from a subset of the OFDMframes, and provide a plurality of radio frames for transmitting asubset of the variable size sub-frames through the radio channel. Theprogram code may also calculate a plurality of timing gaps associatedwith at least one of the variable size sub-frames for providing aprotection for timing variations at signal reception. The timing gap iscalculated based, at least in part, on a cyclic prefix duration of theselected cyclic prefix.

Further features and advantages of the present disclosure, as well asthe structure and operation of various embodiments of the presentdisclosure, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure, in accordance with one or more variousembodiments, is described in detail with reference to the followingFigures. The drawings are provided for purposes of illustration only andmerely depict exemplary embodiments of the disclosure. These drawingsare provided to facilitate the reader's understanding of the disclosureand should not be considered limiting of the breadth, scope, orapplicability of the disclosure. It should be noted that for clarity andease of illustration these drawings are not necessarily made to scale.

FIG. 1 is an illustration of an OFDM/OFDMA mobile radio channeloperating environment.

FIG. 2 is an illustration of principles of an OFDM/OFDMA multicarriertransmission with four subcarriers.

FIG. 3 is an illustration of exemplary OFDM/OFDMA symbols distorted dueto ISI.

FIG. 4 is an illustration of exemplary OFDM/OFDMA symbols with cyclicprefix insertions in the time domain.

FIG. 5 is an illustration of an exemplary cyclic prefix extension to anOFDM/OFDMA symbol in the frequency domain.

FIG. 6 is an illustration of an exemplary OFDM/OFDMA exemplarycommunication system according to an embodiment of the invention.

FIG. 7 is an illustration of an exemplary OFDM/OFDMA digital transceiveraccording to an embodiment of the invention.

FIG. 8 is an illustration of an exemplary OFDM/OFDMA signal definitionin the frequency domain.

FIG. 9 is an illustration of an exemplary OFDM /OFDMA symbol structurein the time domain.

FIG. 10 is an illustration of an exemplary OFDM/OFDMA sub framestructure according to an embodiment of the invention.

FIG. 11 is an illustration of an exemplary OFDM/OFDMA uplink anddownlink radio frame structure according to an embodiment of theinvention.

FIG. 12 is an illustration of an exemplary OFDM/OFDMA uplink anddownlink sub-frame structure according to an embodiment of theinvention.

FIG. 13 is an illustration of an exemplary OFDM/OFDMA optional radioframe structure according to an embodiment of the invention.

FIG. 14 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a n×1.25 MHz bandwidth series according to an embodimentof the invention.

FIG. 15 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a n×3.5 MHz bandwidth series according to an embodimentof the invention.

FIG. 16 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a n×1.25 MHz bandwidth series for 0.5, 0.675, 1, 1.5, 2,and 2.5 ms sub-frames according to an embodiment of the invention.

FIG. 17 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a n×3.5 MHz bandwidth series according to an embodimentof the invention.

FIG. 18 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 0.5 ms sub-frame for a n×1.25 MHz bandwidth series,with a subcarrier frequency spacing Δf=12.5 KHz, according to anembodiment of the invention.

FIG. 19 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 0.675 ms sub-frame for a n×1.25 MHz bandwidth series,with a subcarrier frequency spacing Δf=12.5 KHz, according to anembodiment of the invention.

FIG. 20 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 1.0 ms sub-frame for a n×1.25 MHz bandwidth series,with a subcarrier frequency spacing Δf=12.5 KHz, according to anembodiment of the invention.

FIG. 21 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 1.5 ms sub-frame for a n×1.25 MHz bandwidth series,with a subcarrier frequency spacing Δf=12.5 KHz, according to anembodiment of the invention.

FIG. 22 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 2 ms sub-frame for a n×1.25 MHz bandwidth series, witha subcarrier frequency spacing Δf≈12.5 KHz, according to an embodimentof the invention.

FIG. 23 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 2.5 ms sub-frame for a n×1.25 MHz bandwidth series,with a subcarrier frequency spacing Δf≈12.5 KHz, according to anembodiment of the invention.

FIG. 24 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 5 MHz bandwidth series, with a subcarrier frequencyspacing Δf≈10.94 KHz according to an embodiment of the invention.

FIG. 25 is an illustration of exemplary table of basic OFDM/OFDMAparameters for a 5 MHz bandwidth series, with a subcarrier frequencyspacing Δf≈12.5 KHz, according to an embodiment of the invention.

FIG. 26 is an illustration of an exemplary table of basic OFDM/OFDMAparameters for a 5 MHz bandwidth series, with a subcarrier frequencyspacing Δf≈25 KHz, according to an embodiment of the invention.

FIG. 27 is an illustration of a flowchart showing an OFDM/OFDMA processfor creating a frame structure with a variable cyclic prefix, accordingto embodiments of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description is presented to enable a person of ordinaryskill in the art to make and use the invention. Descriptions of specificdevices, techniques, and applications are provided only as examples.Various modifications to the examples described herein will be readilyapparent to those of ordinary skill in the art, and the generalprinciples defined herein may be applied to other examples andapplications without departing from the spirit and scope of theinvention. Thus, the present invention is not intended to be limited tothe examples described herein and shown, but is to be accorded the scopeconsistent with the claims.

The present disclosure is directed toward systems and methods forOFDM/OFDMA frame structure technology for communication systems.Embodiments of the invention are described herein in the context of onepractical application, namely, communication between a base station anda plurality of mobile devices. In this context, the example system isapplicable to provide data communications between a base station and aplurality of mobile devices. Embodiments of the disclosure, however, arenot limited to such base station and mobile device communicationapplications, and the methods described herein may also be utilized inother applications such as mobile-to-mobile communications, or wirelesslocal loop communications. As would be apparent to one of ordinary skillin the art after reading this description, these are merely examples andthe invention is not limited to operating in accordance with theseexamples.

As explained in additional detail below, the OFDM/OFDMA frame structurecomprises a variable length sub-frame structure with an efficientlysized cyclic prefix operable to effectively utilize OFDM/OFDMAbandwidth. The frame structure provides compatibility with multiplewireless communication systems.

FIG. 6 shows an exemplary wireless communication system 600 fortransmitting and receiving OFDM/OFDMA transmissions in accordance withthe present invention. The system 600 may include components andelements configured to support known or conventional operating featuresthat need not be described in detail herein. In the exemplaryembodiment, system 600 can be used to transmit and receive OFDM/OFDMAdata symbols in a wireless communication environment such as thewireless communication environment 100 (FIG. 1). System 600 generallycomprises a base station transceiver module 602, a base station antenna606, a mobile station transceiver module 608, a mobile station antenna612, a base station processor module 616, a base station memory module618, a mobile station memory module 620, a mobile station processormodule 622, and a network communication module 626.

System 600 may comprise any number of modules other the modules shown inFIG. 6. Furthermore, these and other elements of system 600 may beinterconnected together using a data communication bus (e.g., 628, 630),or any suitable interconnection arrangement. Such interconnectionfacilitates communication between the various elements of wirelesssystem 600. Those skilled in the art will understand that the variousillustrative blocks, modules, circuits, and processing logic describedin connection with the embodiments disclosed herein may be implementedin hardware, computer-readable software, firmware, or any practicalcombination thereof. To clearly illustrate this interchangeability andcompatibility of hardware, firmware, and software, various illustrativecomponents, blocks, modules, circuits, and steps are described generallyin terms of their functionality. Whether such functionality isimplemented as hardware, firmware, or software depends upon theparticular application and design constraints imposed on the overallsystem. Those familiar with the concepts described herein may implementsuch functionality in a suitable manner for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

In the exemplary OFDM/OFDMA system 600, the base station transceiver 602and the mobile station transceiver 608 each comprise a transmittermodule and a receiver module (not shown in FIG. 6). Operation of thetransmitter and receiver modules is explained in more detail in thecontext of the discussion of FIG. 7. For this example, the transmitterand receiver modules are coupled to a shared antenna to form a timedivision duplex (TDD) system. The base station transceiver 602 iscoupled to the base station antenna 606 and the mobile stationtransceiver 608 is coupled to the base station antenna 612. Although ina simple time division duplex (TDD) system only one antenna is required,more sophisticated systems may be provided with multiple and/or morecomplex antenna configurations. Additionally, although not shown in thisfigure, those skilled in the art will recognize that a transmitter maytransmit to more than one receiver, and that multiple transmitters maytransmit to the same receiver. In a TDD system, transmit and receivetiming gaps exist as guard bands to protect against transitions fromtransmit to receive and vice versa. For example, a transmission timinggap (TTG) is designed to separate the downlink transmission periodTTG(DL) from uplink transmission period TTG(UL). The downlink TTG(DL)provides a protection for timing variations at signal reception indownlink transmission. The TTG(DL) portion of timing gap is also used toprevent the downlink radio signal colliding with uplink signals due topropagation delay. The TTG(UL) portion of timing gap is used to offsetuplink radio signal propagation delay so that all uplink signalssynchronized at the base station (BS) receiver(s). The TTG(DL) may allowsufficient time for a TDD system to transition from a downlink to anuplink. Similarly, a TTG for the uplink TTG(UL) may allow sufficienttime for a TDD system to transition from an uplink to a downlink.According to an embodiment of the invention, the TTG(DL) and TTG(UL) canbe calculated based on the cyclic prefix duration as explained in moredetail in the context of discussing of FIG. 14.

In the particular example of the OFDM/OFDMA system depicted in FIG. 6,an “uplink” transceiver 608 includes an OFDM/OFDMA transmitter thatshares an antenna with an uplink receiver. A duplex switch mayalternatively couple the uplink transmitter or receiver to the uplinkantenna in time duplex fashion. Similarly, a “downlink” transceiverincludes an OFDM/OFDMA receiver which shares a downlink antenna with adownlink transmitter. A downlink duplex switch may alternatively couplethe downlink transmitter or receiver to the downlink antenna in timeduplex fashion. The operation of the two transceivers 602/608 iscoordinated in time such that the uplink OFDM/OFDMA receiver is coupledto the uplink antenna 612 for reception of transmissions over thewireless transmission link 614 at the same time that the downlinkOFDM/OFDMA transmitter is coupled to the downlink antenna 606.Preferably there is close time synchronization with only a minimal guardtime between changes in duplex direction.

Although many OFDM/OFDMA systems will use OFDM/OFDMA technology in bothdirections, those skilled in the art will recognize that the presentembodiments of the invention are applicable to systems using OFDM/OFDMAtechnology in only one direction, with an alternative transmissiontechnology (or even radio silence) in the opposite direction.Furthermore, it should be understood by a person of ordinary skill inthe art that the OFDM/OFDMA transceiver modules 602/608 may utilizeother communication techniques such as, without limitation, a frequencydivision duplex (FDD) communication technique.

The mobile station transceiver 608 and the base station transceiver 602are configured to communicate via a wireless data communication link614. The mobile station transceiver 608 and the base station transceiver602 cooperate with a suitably configured RF antenna arrangement 606/612that can support a particular wireless communication protocol andmodulation scheme. In the exemplary embodiment, the mobile stationtransceiver 608 and the base station transceiver 602 are configured tosupport industry standards such as the Third Generation PartnershipProject Long Term Evolution (3GPP LTE), Third Generation PartnershipProject 2 Ultra Mobile Broadband (3Gpp2 UMB), Time Division-SynchronousCode Division Multiple Access (TD-SCDMA), and Wireless Interoperabilityfor Microwave Access (WiMAX), and the like. The mobile stationtransceiver 608 and the base station transceiver 602 may be configuredto support alternate, or additional, wireless data communicationprotocols, including future variations of IEEE 802.16, such as 802.16e,802.16m, and so on. In an exemplary embodiment, a mobile stationtransceiver 608 may be used in a user device such as a mobile phone.Alternately, the mobile station transceiver 608 may be used in apersonal digital assistant (PDA) such as a Blackberry device, Palm Treo,MP3 player, or other similar portable device. In some embodiments themobile station transceiver 608 may be a personal wireless computer suchas a wireless notebook computer, a wireless palmtop computer, or othermobile computer devices. In further embodiments, the invention can beimplemented in a mobile station as well as a base station. Thetransmitter at the mobile station can add the variable length cyclicprefixes and understand the changes of the timing gaps accordingly.However, in the current intended practices, the dynamic configuration ofthe variable length cyclic prefixes of mobile stations is set by thebase station. The mobile stations can negotiate with the base stationfor the preferred cyclic prefix. The base station can assign it to aparticular uplink sub-frame to transmit with the preferred cyclicprefix.

Processor modules 616/622 may be implemented, or realized, with ageneral purpose processor, a content addressable memory, a digitalsignal processor, an application specific integrated circuit, a fieldprogrammable gate array, any suitable programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof, designed to perform the functions described herein.In this manner, a processor may be realized as a microprocessor, acontroller, a microcontroller, a state machine, or the like. A processormay also be implemented as a combination of computing devices, e.g., acombination of a digital signal processor and a microprocessor, aplurality of microprocessors, one or more microprocessors in conjunctionwith a digital signal processor core, or any other such configuration.Processor modules 616/622 comprise processing logic that is configuredto carry out the functions, techniques, and processing tasks associatedwith the operation of OFDM/OFDMA system 600. In particular, theprocessing logic is configured to support the OFDM/OFDMA frame structureparameters described herein. For, example the processor modules 616/612may be suitably configured to compute cyclic prefix durations and timingtransitions (TDD (UL) and TDD (DL)), as explained below, to provide aflexible size frame structure. For example, a frame may be constructedfrom one or multiple sub-frames, each sub-frame is consisted of one ormultiple symbols and timing gaps. A timing gap is the period of idletransmission time, such as TTG(DL), TTG(UL), or RTG. Based on this newdefinition, the gap time periods, TTG and RTG, have been included in thesub-frames. This way to define a frame and sub-frame (also known as“slot” in LTE) has greatly simply the design of a frame, and make itmuch more flexible for different sub-frame designs. The newly definedsub-frame has been self-contained within its time period and boundary.Sub-frames with different cyclic prefixes can co-exist in the samesystem and the same deployment. As mentioned above, in some embodimentsthe processing logic may be resident in the base station and/or may bepart of a network architecture that communicates with the base stationtransceiver 602.

Furthermore, the steps of a method or algorithm described in connectionwith the embodiments disclosed herein may be embodied directly inhardware, in firmware, in a software module executed by processormodules 616/622, or in any practical combination thereof. A softwaremodule may reside in memory modules 618/620, which may be realized asRAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory,registers, a hard disk, a removable disk, a CD-ROM, or any other form ofstorage medium known in the art. In this regard, memory modules 618/620may be coupled to the processor modules 618/622 respectively such thatthe processors modules 616/620 can read information from, and writeinformation to, memory modules 618/620. As an example, processor module616, and memory modules 618, processor module 622, and memory module 620may reside in their respective ASICs. The memory modules 618/620 mayalso be integrated into the processor modules 616/620. In an embodiment,the memory module 618/620 may include a cache memory for storingtemporary variables or other intermediate information during executionof instructions to be executed by processor modules 616/622. Memorymodules 618/620 may also include non-volatile memory for storinginstructions to be executed by the processor modules 616/620.

Memory modules 618/620 may include a frame structure database (notshown) in accordance with an exemplary embodiment of the invention.Frame structure parameter databases may be configured to store,maintain, and provide data as needed to support the functionality ofsystem 600 in the manner described below. Moreover, a frame structuredatabase may be a local database coupled to the processors 616/622, ormay be a remote database, for example, a central network database, andthe like. A frame structure database may be configured to maintain,without limitation, frame structure parameters as explained below. Inthis manner, a frame structure database may include a lookup table forpurposes of storing frame structure parameters.

The network communication module 626 generally represents the hardware,software, firmware, processing logic, and/or other components of system600 that enable bi-directional communication between base stationtransceiver 602, and network components to which the base stationtransceiver 602 is connected. For example, network communication module626 may be configured to support internet or WiMAX traffic. In a typicaldeployment, without limitation, network communication module 626provides an 802.3 Ethernet interface such that base station transceiver602 can communicate with a conventional Ethernet based computer network.In this manner, the network communication module 626 may include aphysical interface for connection to the computer network (e.g., MobileSwitching Center (MSC)).

FIG. 7 is a block diagram of an exemplary OFDM/OFDMA transceiver system700 (e.g., transceivers 602 or 608 in FIG. 6) that can be configured inaccordance with an exemplary embodiment of the invention. FIG. 7represents a method for adding an efficiently sized cyclic prefix to anOFDM/OFDMA frame structure operable to effectively utilize OFDM/OFDMAchannel transmission bandwidth. It is understood that, the system 700may include additional components and elements configured to supportknown or conventional operating features. For the sake of brevity,conventional techniques and components related to digital signalprocessing such as channel encoding/decoding, correlation techniques,spreading/dispreading, pulse shaping, radio frequency (RF) technology,and other functional aspects and the individual operating components ofthe system 700 are not described in detail herein. The OFDM/OFDMA system700 digitally transmits and receives data wirelessly to and frominfrastructure devices using IFFT/FFT techniques. A discrete Fouriertransform (DFT) and an inverse discrete Fourier transform (IDFT) may beused as an alternative to an FFT and IFFT respectively.

The OFDM/OFDMA digital transceiver system 700 includes a transmitter 701and a receiver 703. The transmitter 701 further includes aserial-to-parallel converter 702, an OFDM/OFDMA module 704, and adigital-to-analog converter (D/A) module 712. The OFDM/OFDMA module 704includes an IDFT/IFFT module 706, a parallel-to-serial converter 708,and an add cyclic prefix module 710 coupled to a cyclic prefix selector709. The receiver 703 includes an analog-to-digital converter (A/D)module 716, an inverse OFDM/OFDMA receiver module 718, and aparallel-to-serial converter 726. The inverse OFDM/OFDMA receiver module718 includes a remove cyclic prefix module 720, a serial-to-parallelconverter 722, and a DFT/FFT module 724. In this example, thetransmitter 701 and the receiver 703 can send and receive data and othercommunication signals via a multipath propagation channel 714 or otherchannels (e.g., 614 in FIG. 6).

In the transmitter 701, a serial stream of N_(C) source data symbolsD_(n) (corresponding to serial data symbols 202 in FIG. 2) is convertedinto N_(C) parallel data symbols (corresponding to parallel data symbols210 in FIG. 2) by the serial-to-parallel converter 702. The source datasymbols D_(n) may, for example, be obtained from an original data source(e.g., a text message) after source and channel coding, interleaving,and symbol mapping. After serial to parallel conversion 702, the sourcedata symbol duration T_(d) of the N_(C) serial data symbols results inthe OFDM/OFDMA symbol duration T_(u)=N_(C)*T_(d).

The parallel data symbols are then modulated on to N_(C) differentsub-carriers (206 in FIG. 2) via the IDFT/IFFT module 706. As explainedabove, an OFDM system modulates the N_(C) parallel data sub-streams onto N_(C) sub-carriers (206 in FIG. 2). The N_(C) sub-carriers have afrequency spacing of

${\Delta \; f} = \frac{1}{T_{u}}$

in order to achieve orthogonality between the signals on the N_(C)sub-carriers. The N_(C) parallel modulated signals form an OFDM symbol(210 in FIG. 2). Depending on the transmission media and the networkbandwidth, system 700 can employ, for example, 64 to 4096 subcarriers asexplained in more detail below.

The add cyclic prefix module 710 is then used to add a cyclic prefix tothe output of the parallel-to-serial converter 708. In order tocompletely avoid or significantly reduce the effects of ISI, a cyclicprefix of duration T_(G) may be inserted between adjacent OFDM/OFDMAsymbols (402 in FIG. 4). The cyclic prefix duration parameter T_(G) maybe set to various values in order to efficiently size the cyclic prefixto effectively utilize OFDM/OFDMA bandwidth. As explained above, acyclic prefix is a cyclic extension of each OFDM symbol which isobtained by extending the duration of an OFDM symbol toT_(SYM)=T_(u)+T_(G) in accordance with one embodiment of the invention.

For example, according to an embodiment of the invention, the cyclicprefix values are selected based on RF channel conditions. In thismanner, the cyclic prefix is configurable even if a system is deployed,thereby allowing efficient use of bandwidth. Accordingly, acommunication system can select various effective cyclic prefix lengthsfor the base stations in a network, and may support different cyclicprefix lengths for different base stations in the network. Furthermore,a communication system may support different cyclic prefix lengths indifferent downlink and/or uplink sub-frames for the same base station. Avariable cyclic prefix length allows a base station to change orconfigure the cyclic prefix duration for different communication usagescenarios, thereby increasing the bandwidth efficiency (bit/Hz) of thesystem. For example, when communication is in a channel with a severemultipath (i.e., larger delay spread), longer cyclic prefixes can beused to eliminate the ISI and ICI. In less severe channel conditionswith fewer multipaths, a short cyclic prefix can be used in order toincrease data rate (bits/sec), and to reduce overhead and transmissionpower.

The add cyclic prefix module 710 may receive the cyclic prefix valuesfrom the cyclic prefix selector 709. The cyclic prefix selector 709 maycommunicate with the processor 622 and or the memory module 620 toobtain values for the cyclic prefix. For example, the value maycorrespond to the cyclic prefix duration needed for base stationcoverage. The cyclic prefix selector 709 will then provide theappropriate cyclic prefix value to the add cyclic prefix module 710. Forexample, the cyclic prefix duration can be associated with multipathdelay spread. When a Femto base station is deployed, for example, due tolow transmit power of the base station power amplifier (PA) it onlycovers a small area or a hot-spot. The delay spread becomes very smalland therefore this Femto BS and the associated mobile stations shouldselect a small cyclic prefix (out of the different length options) forthe downlink and uplink sub-frames transmission. On the other hand, if aMacro base station is deployed, due to the large transmit power of thebase station power amplifier (PA), the base station can group theserving mobile stations into different groups by their delay spreads(determined by base station or requested by individual mobile station).The base station can allocate these different groups of mobile stationsinto different sub-frames with appropriate settings of cyclic prefixes.The cyclic prefix is just a copy of the end portion of the useful symbol(T_(u)), it is calculated and copied on the fly. The cyclic prefixvalues may be chosen to efficiently size the cyclic prefix toeffectively utilize OFDM/OFDMA bandwidth, while providing a framestructure compatible with multiple wireless communication systems. Thespecific cyclic prefix values that are used are discussed below in thecontext of discussion of FIGS. 14-26.

The output of the add cyclic prefix module 710 is then passed throughthe D/A 712 to create an analog signal for transmission. The output ofD/A 712 comprises the signal waveform X(t) with duration T_(SYM). Thesignal waveform X(t) is up converted (not shown) and the RF signal istransmitted to the channel 714.

The output of the channel 714, after RF down conversion (not shown), isthe received signal waveform Y(t) which may include ISI from the channeland RF processing. The received signal Y(t) is passed throughanalog-to-digital convert 716, whose output sequence Y_(v) is thereceived signal Y(t) sampled at rate

$\frac{1}{T_{d}}.$

A sampling rate of

$\frac{1}{T_{d}}$

or greater is necessary to insure proper Nyquist sampling for the datarate of the data symbols D_(n) with duration T_(d).

Since ISI may be present within the cyclic prefix time period, cyclicprefix samples are removed via remove cyclic prefix module 720 beforeDFT/FFT demodulation. The ISI-free part of Y(t) may be converted toparallel data symbols via serial-to-parallel converter 722, anddemodulated by DFT/FFT module 724. The output of the DFT/FFT module 724is a sequence R_(n), which is the received replica of the original datasymbols D_(n) along with any transmission errors. The receiver mayincorporate other techniques which are not illustrated here, such aschannel estimation, maximum receive ratio combining, etc.

FIG. 8 is an illustration of an exemplary OFDM/OFDMA signal frequencydomain definition 800. As will be explained below, the choice ofsub-carrier frequency in the OFDM/OFDMA signal frequency domain may beused, according to an embodiment of the invention, to insure frequencycompatibility with wireless communication standards. The OFDM/OFDMAsignal frequency domain definition 800 may comprise a nominal channeltransmission bandwidth (BW) 802, a subset of signal subcarriers(N_(SIG)) 804, a signal bandwidth (BWS_(SIG)) 808, a DC sub-carrier (DC)810, guard subcarriers 812, and a sampling frequency (F_(S)) 814. Insome systems, the DC sub-carrier may not be defined.

For a given nominal channel transmission bandwidth (BW) 802, a subset ofsignal subcarriers 804 out of a plurality of subcarriers 806 may be usedto match the bandwidth of the subcarriers 804 to the channeltransmission bandwidth BW 802. The subset of signal subcarriers 804 isreferred as signal bandwidth (BW_(SIG)) 808. The plurality ofsubcarriers 806 may include the DC sub-carrier (DC) 810, which containsno data. Subcarriers outside the signal bandwidth (BW_(SIG)) 808 thatare not used may serve as guard subcarriers 812. The purpose of theguard subcarriers 812 is to enable the signal to have a smooth roll offin the time domain.

FIG. 9 is an illustration of an exemplary time domain symbol structureof an OFDM/OFDMA signal. FIG. 9 illustrates the positioning of a cyclicprefix in the exemplary time domain OFDM symbol. The time domain symbolstructure 900 comprises a useful symbol time (T_(u) or T_(IFFT)) 902, acyclic prefix 904, a windowing period (T_(WIN)) 908, and a total symboltime (T_(SYM)) 910. In some systems, the windowing period may not bedefined, which can be treated as windowing period having a value ofzero: T_(WIN)=0.

A time duration of a set of OFDM data symbols to be transmitted by anOFDM/OFDMA system is referred to as the useful symbol time (T_(u) orT_(IFFT)) 902. A copy of the end section of the symbol period 906 isused to produce the cyclic prefix (CP) 904. By using a cyclic extension,the samples used to perform the FFT at the receiver can be takenanywhere over the length of the extended symbol. This provides multipathimmunity as well as a tolerance for symbol time synchronization errors.A small windowing period (T_(WIN)) 908 can be added before the cyclicprefix 904 and at the end of symbol time 902 to reduce signal in-bandand out-of-band emission. In this example, the total symbol time(T_(SYM)) 910 may include the useful symbol time (T_(u) or T_(IFFT))902, the cyclic prefix duration T_(G) 904, and a windowing period(T_(WIN)) 908. An inverse Fourier transform (IFFT) of a set of OFDM datasymbols in the time duration T_(SYM) creates an OFDM/OFDMA waveform.

As explained above, many interference sources such as ISI, ICI, andmultipath, can have an effect on OFDM/OFDMA system performance.Furthermore, the choice of the frame structure and the parameters thatdefine the frames may also determine the performance of an OFDM/OFDMAsystem. A tradeoff must generally be made between resistance tointerference and data transmission capacity. The tradeoff is determinedby the choice of the parameters of the radio frames. For example, a longcyclic prefix may improve the multipath performance, but reduce theoverall throughput of the system and overall frequency efficiency.

FIG. 10 is an illustration of an exemplary OFDM/OFDMA sub-framestructure according to an embodiment of the invention. For this example,the OFDM/OFDMA sub-frame structure comprises a short sub-frame 1002, aregular sub-frame 1004, a long sub-frame 1006, and an optional low chiprate (LCR) sub-frame 1008. A 10 ms radio frame may be divided intotwenty or more short sub-frames 1002, ten regular sub-frames 1004, orfive long sub-frames 1006. For a 10 ms radio frame divided in this way,a short sub-frame 1002 has a duration of 0.5 ms, a regular sub-frame1004 has a duration of 1 ms, and a long sub-frame 1006 has a duration of2 ms. Other numbers of sub-frames that don't necessarily divide the 10ms radio frame evenly may also be used. In this case, a gap remains inthe radio frame. For example, a long sub-frame may have six longsub-frames each with a duration of 1.5 ms. Then, the total time of thesub-frames is 9 ms, which leaves a gap of 1 ms in the radio frame. Theoptional low chip rate sub-frame 1008 may also be used. A low chip ratesub-frame 1008 may have a duration of 0.675 ms, and a 10 ms radio framemay be divided into 14 or more low chip rate sub-frames 1008 with a 0.55ms gap. These sub-frame duration options may allow a communicationsystem such as the system 600 to reduce interference with other systemsthat are based on various industry standards as mentioned in the contextof FIG. 6 above.

The frame structure provides compatibility with multiple wirelesscommunication systems. For example, the low chip rate sub-frame 1008duration of 0.675 ms may allow compatibility with the TimeDivision-Synchronous Code Division Multiple Access (TD-SCDMA) OFDM/OFDMAradio frame structure. The long sub-frame 1006 duration of 2 ms mayallow compatibility with the Third Generation Partnership Project LongTerm Evolution (3GPP LTE) OFDM/OFDMA radio frame structure, and thelike.

FIG. 11 is an illustration of an exemplary OFDM/OFDMA radio framestructure 1100 according to an embodiment of the invention. TheOFDM/OFDMA radio frame structure 1100 may include five exemplary framestructures 1102, 1104, 1106, 1108, and 1112. The sub-frames may beallocated for uplink or downlink transmission. The first frame structure1102 illustrates a series of alternating uplink regular sub-frames(shown by arrows pointing up) and downlink regular sub-frames (shown byarrows pointing down). The second exemplary frame structure 1104illustrates a series of alternating uplink short sub-frames and downlinkshort sub-frames. This can give the same uplink data rate and downlinkdata rate, but in contrast to the first exemplary frame structure 1102,the second exemplary frame structure 1104 would have a lower overalldata rate (bits/sec) because of an increase in overhead, and a lowerlatency because of the delay between sub-frames. Lower latency is usefulfor some applications like vocoders, where time delay is critical. Thethird exemplary frame structure 1106 illustrates a series of alternatinguplink regular sub-frames and downlink long sub-frames. This can give agreater downlink data rate than uplink data rate. The fourth exemplaryframe structure 1108 illustrates a series of alternating uplink shortsub-frames and downlink long sub-frames. This can give an even greaterdownlink data rate than uplink data rate. The fifth exemplary framestructure 1110 illustrates a series of alternating downlink longsub-frames with a uplink short sub-frame and a downlink short sub-frame.This would be useful, for applications like an internet download, wheresmall control commands alternate with large webpage downloads. In manyreal world situations, and particularly for multiple access systems likeOFDMA, the frame synchronization on uplinks and downlinks to the variouscommunicating devices may have timing variations.

FIG. 12 is an illustration of an exemplary OFDM/OFDMA uplink anddownlink frame structure 1200 according to an embodiment of theinvention. The OFDM/OFDMA uplink and downlink frame structure 1200includes a downlink sub-frame 1202, a single uplink sub-frame 1204, alast uplink sub-frame 1206, and an uplink sub-frame 1208. Each sub-frame1202/1204/1206/1208 includes a plurality of symbols 1210.

The downlink sub-frame 1202 has a transmission timing gap (TTG) for thedownlink (TTG(DL)) 1212. Transmit and receive timing gaps exist as guardperiods to protect against transitions from transmit to receive and viceversa in a TDD system. The TTG(DL) 1212 provides a protection for timingvariations at signal reception, and allows sufficient time for a TDDsystem to transition from a downlink to an uplink. A TTG(DL) is aportion of the transmit/receive transition gap contributed from thedownlink sub-frame.

For the single uplink sub-frame 1204, there is a transmission-timing gapat each end of the transmission for single uplink sub-frame 1204, sincea single uplink sub-frame 1204 is both the start and end of its seriesin a frame. The single uplink sub-frame 1204 has a transmission timinggap for the uplink (TTG(UL)) 1214 and a receive-transmit transition gap(RTG) 1216 according to an embodiment of this invention. The TTG(UL)1214 and RTG 1216 provide a protection for timing variations at signalreception, and the TTG(UL) 1214 allows sufficient time for a TDD systemto transition from a downlink to an uplink. A TTG(UL) is the portion ofthe transmit/receive transition gap contributed from the uplinksub-frame. The RTG 1216 allows a TDD system (FIG. 6) time to transitionfrom an uplink back to a downlink. Since the necessary timing gap periodfor RTG is often very short, it is optional in the system design. Insome systems, RTG can be set to 0. In theory the base station may takeup very small portion of the cyclic prefix time for switching fromtransmitting to receiving mode, but it is typically up to the basestation to adjust when the uplink frame starts. In one embodiment, theuplink frame is sent in advance in time to offset propagation delay,therefore there is more than sufficient time for the mobile station toswitch from transmitting mode to receiving mode without sacrificing thecyclic prefix for transaction. When RTG is set to 0, then the systemdesign is further simplified, then “single uplink sub-frame” 1204 and“last uplink sub-frame” 1206 become the same design as “uplinksub-frame” with only TTG(UL) 1208.

If there is a series of uplink sub-frames in a frame, then uplinksub-frame 1208 starts the series and the last uplink sub-frame 1206 endsthe series. The uplink sub-frame 1208 begins the series with a TTG(UL)1214, which provides a time gap to allow a TDD radio system base stationto transition from transmit mode to receive mode, and a TDD radio systemmobile station to transition from receive mode to transmit mode. Afterthe transition, subsequent sub-frames transmitted on the uplink may besub-frames without time gaps. For the last sub-frame in the uplinkseries, a last uplink sub-frame 1206 is transmitted as explained above,which ends the series with the RTG 1216. The RTG 1216 provides a timegap to allow a TDD radio system base station to transition from receivemode to transmit mode, and a TDD radio system mobile station totransition from transmit mode to receive mode. According to anembodiment of the invention values for the TTG(DL) and TTG (UL) can becalculated based on the cyclic prefix duration as explained below in thecontext of discussion of FIG. 14.

FIG. 13 is an illustration of an exemplary OFDM/OFDMA optional radioframe 1300 according to an embodiment of the invention. The optionalradio frame 1300 is 5 ms in length 1302. It starts with a 0.675 msoptional sub-frame 1304. Then a 75 μs downlink pilot (DwPTS) 1306 istransmitted. A 75 μs gap period (GP) 1308 is allowed betweentransmissions, and then a 125 μs transmitted uplink pilot (UpPTS) 1310is transmitted. Then 0.675 ms optional sub-frame 1312 is transmitted upto the end of the frame 1300. In one embodiment of the invention, theDwPTS, GP, UpPTS are used to provide downlink and uplink transmissionperiods that are synchronized/lined-up with the TD-SCDMA for adjacent RFchannel deployment.

FIGS. 14-26 illustrate exemplary tables of basic OFDM/OFDMA parametersfor several channel transmission bandwidth series according to variousembodiments of the invention. The OFDM/OFDMA parameters detail thevariable length sub-frame parameters of the OFDM/OFDMA frame structure.As explained above, the frame structure may provide compatibility withmultiple wireless communication systems using an efficiently sizedcyclic prefix to efficiently utilize OFDM/OFDMA bandwidth. Note thatnumerology specified in these tables is for exemplary purposes only andother values for the OFDM/OFDMA parameters may be used.

FIG. 14 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a n×1.25 MHz bandwidth series according to an embodiment of theinvention. A n×1.25 bandwidth series includes channel transmissionbandwidths of 1.25, 2.5, 5, 10, 20, and 40 MHz based on multiples of1.25 MHz. FIG. 14 shows sub-frame duration, subcarrier spacing, samplingfrequency, FFT size N_(FFT), number of occupied subcarriers, number ofOFDM/OFDMA symbols per sub-frame, cyclic prefix durations of each of thesub-frames, and the cyclic prefix duration of the TTG(DL), TTG(UL), andRTG of each the sub-frames.

The FFT size N_(FFT) may be the smallest power of two that is greaterthan the required number of signal subcarriers (804 in FIG. 8) neededfor the sampling frequency F_(S) (814 in FIG. 8) for the OFDM/OFDMAsystem. For example, for a transmission BW of 1.25 MHz, and a carrierspacing Δf=12.5 kHz, the required number of signal subcarriers (804 inFIG. 8) can be 100. Then, the FFT size N_(FFT) is equal to 128 which isthe smallest power of two (i.e., 2⁷) that is less than 100.

In this example, the FFT size N_(FFT) is scalable from 128 to 4096. Whenthe available channel transmission bandwidth BW increases, the N_(FFT)also increases such that Δf is constant. This keeps the OFDM/OFDMAsymbol duration T_(u) fixed, which is independent of channel systembandwidth BW. T_(SYM) (T_(SYM)=T_(u)+T_(G)) is configurable based ondifferent deployment scenarios, and therefore makes scaling have aminimal impact on higher layers. For example, a 7 MHz system may havethe same performance as a 10 MHz system, except for that the maximumdata throughput is proportional to the channel bandwidth (BW). A 5 MHzsystem can migrate to a 10 MHz system by adding another 5 MHz channel BWright next to it without a guard band and without causing adjacentchannel interference by simply making all subcarriers orthogonal to eachother. The migration can be done with the same base station and mobilestation, as long the bandwidth filter has been designed for a 10 MHzchannel. All frequency bands and rasters (200 KHz and 250 KHz) in theworld can be divided by 12.5 KHz evenly, with no extra bandwidth andbanding constraints. A scalable design also keeps the costs low.

In this example embodiment, an OFDM/OFDMA system with a fixed subcarrierspacing value Δf=12.5 kHz may be used. The Δf=12.5 kHz is chosen becauseit can not only divide the common channel raster of 200 KHz evenly, butalso divide the alternative common channel raster of 250 KHz evenly.Thus, a frequency spacing of Δf=12.5 kHz can divide all RF channelevenly without unnecessary residue bandwidth. Additionally, the adjacentbands that are adopting the same technology will have minimuminter-channel interference (ICI), simply all adjacent sub-carriers areorthogonal to each other. Similarly Δf=10 kHz, 20 kHz, 25 kHz can servethe same purpose. The higher the Δf is selected the higher the Dopplershift, often caused by mobility, the system can tolerate. As mentionedabove, in the frequency domain an OFDM or OFDMA signal is made up oforthogonal subcarriers, and the number of used subcarriers may be lessthan or equal to the FFT size (N_(FFT)). For example, the FFT size(N_(FFT)) may be in a range comprising 128, 256, 512, 1024, 2048, or4096 subcarriers.

The sampling frequency (e.g., F_(S)=1.6, 3.2, 6.4, 12.8, 25.6, and 51.2MHz) can be calculated based on the N_(FFT) and Δf using the followingequation:

F _(S) =Δf×N _(FFT)

With this particular subcarrier spacing, RF channels with differentchannel transmission bandwidths are scalable. They can be defined withaccordant used subcarriers within a fast Fourier transform size N_(FFT).A subcarrier spacing of Δf=12.5 kHz has a property of good trade-off ofcyclic prefix overhead with mobility support and achieving reasonablefrequency efficiency.

For a given nominal channel transmission bandwidth BW (802 in FIG. 8)only a subset of subcarriers N_(SIG) out of N_(FFT) is occupied forsignal bandwidth BW_(SIG). For example, according to an embodiment ofthe invention the number of occupied subcarriers for a channeltransmission bandwidth BW of 1.25, 2.5, 5, 10, 20, and 40 MHz can be 20,100, 200, 400, 800, 1600 and 3200 respectively.

As explained above, in addition to the useful symbol duration T_(u)which is available for user data transmission, an additional period oftime T_(G) can be used for transmission of a cyclic prefix. The cyclicprefix duration is prepended to each useful symbol duration T_(u) and isused to compensate for the dispersion introduced by the channel responseand by the pulse shaping filter used at the transmitter. Thus, althougha total OFDM/OFDMA symbol duration of T_(SYM)=T_(u)+T_(G)+T_(WIN) isemployed for transmitting an OFDM symbol, the useful symbol duration:

$T_{u} = \frac{1}{\Delta \; f}$

is available for user data transmission. T_(u) is therefore called theuseful OFDM/OFDMA symbol duration. The windowing time period T_(WIN) isoptional, it can be set to 0 in some communication systems, such as theIEEE 802.16e version of WiMAX.

As mentioned above, in the existing systems the cyclic prefix isconfigurable, but it is fixed when a system is deployed, therebyconstraining system configuration for efficient bandwidth utilization.In these existing systems, cyclic prefix length may not be variable andone type of cyclic prefix may exist. In this manner, existing systemsmay not allow a base station to change or configure the cyclic prefixduration to adjust to varying channel conditions.

According to one embodiment of the invention, for example, when acommunication channel has a severe multipath delay spread (i.e., largerdelay spread), a longer cyclic prefix duration can be used to eliminatethe ISI. In a less severe channel conditions with less multipath delayspread, a short cyclic prefix can be used in order to reduce radiooverhead and improve overall throughput and spectral efficiency. In thismanner, various cyclic prefix types may be used for small, regular, andlarge cell site deployment as explained in more detail below. Thevarious cyclic prefix types are referred to as Short, Normal, and Longrespectively.

A cyclic prefix duration T_(G) may be calculated based on the followingrelationship:

${T_{G} = \frac{C\; P_{samples}}{F_{S}}},$

where F_(S) is the sampling frequency as shown above and CP_(samples) isthe number of samples per cyclic prefix. Where, CP_(samples)=T_(G)×F_(S)can be obtained from knowing T_(G) and F_(S). The value of T_(G) isselected in the particular sub-frame configuration, such as Short,Normal, or Long.

An initial T_(G) (plus T_(WIN)) value may be selected for an initialchannel. As shown in FIG. 14, at a 10 MHz channel one uplink sub-framecan be configured for Normal Cyclic Prefix (Normal CP), T_(G)=10 μs, inwhich CP_(samples) may comprise 128 samples. Another uplink sub-framecould be configured for Short Cyclic Prefix (Short CP), T_(G)=3.125 μs,in which CP_(samples) may comprise only 40 samples. For example, aninitial Normal T_(G)=10 μs is selected based on a typical cell sitecoverage, and the downlink control channel, multicast and broadcastsub-frames and the Normal T_(G) is used so that all mobile stations areable to listen to the base station. However, once the base station hasidentified that one or multiple mobile stations are close enough to thebase station with small delay spreads, the base station can allocatethese mobile stations to transmit in the uplink sub-frames with ShortCyclic Prefix (Short CP) T_(G)=3.125 μs. The base station can alsotransmit the unicast or multicast information so these mobile stationscan also use a downlink sub-frame with Short Cyclic Prefix (Short CP)T_(G)=3.125 μs. For example, for a channel transmission bandwidth of 2.5MHz and a Short cyclic prefix, a CP_(samples) value of 10 (3.125×3.2)may be obtained. In this manner, for a given bandwidth series,CP_(samples) may be scaled by the sampling frequencies so as to keep thecyclic prefix duration T_(G) constant. For example, for a Short cyclicprefix, CP_(samples) may be 5/10/20/40/80/160 for channel transmissionbandwidth of 1.25/2.5/5/10/20/40 MHz respectively, while T_(G) remainsat 3.125 μs. A system with different bandwidths will have the sameperformance and user experience. In IEEE 802.16e version of WIMAX, asubscriber moves from a 7 MHz system to a 10 MHz system, T_(G) isreduced accordingly with the bandwidth increase. The same subscriber mayexperience more dropped calls in the 10 MHz system. It has imposed greatconstraints on the cell site planning, and it is hard to maintain thesame user experience across different bandwidth systems.

Alternatively, variable cyclic prefix durations may be chosen for agiven channel transmission bandwidth based on the same sub-carrierspacing Δf=12.5 kHz. For example, cyclic prefix durations ofT_(G)≈3.125/10/16.875 μs may be chosen for the Short, Normal, and Longcyclic prefixes respectively. These cyclic prefix durations can be usedfor small, regular, and large cell site deployment as explained above.Selecting different cyclic prefixes for OFDM/OFDMA symbols in asub-frame for a base station allows for supporting different types ofbase station cells and cell coverage areas. Thereby, network deploymentmay be simplified by eliminating the need for the entire network toselect the same cyclic prefixes regardless of the different requirementson each base station for its cell coverage area.

For a TDD system, downlink and uplink radio propagation are reciprocal,the base stations can detect and determine whether a smaller size ofcyclic prefix is sufficient for a particular mobile station. On theother hand, a mobile station also can measure the downlink signals froma base station to determine what size of cyclic prefix is sufficient forthe uplink transmission. The mobile station can report to the basestation the preferred size of the cyclic prefix.

Different sub-frame (e.g., Short, Reg., Long) durations T_(Sub-frame)can be designed based on different cyclic prefix durations such asShort, Normal, and Long cyclic prefix durations. For example, in FIG. 14sub-frame durations may be T_(Sub-frame)=0.5, 1, and 1.5 ms for theShort, Regular, and Long durations respectively. Also in FIG. 14, thecyclic prefix duration T_(G)+T_(WIN) for a 1.25 MHz channel transmissionbandwidth may be approximately 3.125/10/16.875 μs for Short, Normal, andLong cyclic prefix durations respectively. Accordingly, useful bandwidthcan be allocated for data transmission instead of cyclic prefixtransmission, thereby increasing the bandwidth efficiency (bits/Hz). Inthis manner, the overhead from the cyclic prefix duration can beminimized.

For a given T_(Sub-frame) and cyclic prefix type, the number ofOFDM/OFDMA symbols per sub-frame (N_(SYM)) can be a function of thesampling frequency F_(S) and FFT size N_(FFT). F_(S) and N_(FFT) may bechosen so that N_(SYM) can remain the same across the bandwidth series.For example, for a 1.25 MHz transmission bandwidth N_(SYM) may becalculated based on the following relationship:

${N_{SYM} < \frac{{T_{{Sub}\text{-}{frame}} \times F_{s}} - {\frac{1}{2}T\; T\; G_{\min \mspace{14mu} {samples}}}}{N_{F\; F\; T} + {C\; P_{sample}}}} = {\frac{{1.5 \times 1000 \times 1.6} - 5}{128 + 5} = 18.01}$

When the carrier spacing Δf is fixed, the length of the useful symbolduration becomes constant,

$T_{u} = {\frac{1}{\Delta \; f}.}$

For a given cyclic prefix duration, the length of symbol duration isalso determined (assuming T_(WIN)=0),

$T_{SYM} = {{T_{u} + T_{WIN} + T_{G}} = {\frac{1}{\Delta \; f} + {T_{G}.}}}$

For a given sub-frame duration T_(Sub-Frame), the T_(Sub-Frame)comprises transmission time and idle time. The transmission time isoccupied by radio signal of multiple of symbols, N_(SYM)×T_(SYM). Theleftover idle time is used for transmit transition gap (TTG) timeTTG_(Sub-Frame) and receive transition gap (RTG) time RTG_(Sub-Frame),the latter is typically applicable to only uplink sub-frame. The valueof RTG is often small. The number of OFDM/OFDMA symbols per sub-frame(^(N) ^(SYM) ) can be calculated as following:

$N_{SYM} = \frac{T_{{Sub}\text{-}{frame}} - {T\; T\; G_{{Sub}\text{-}{frame}}} - {R\; T\; {G\;}_{{Sub}\text{-}{frame}}}}{\frac{1}{\Delta \; f} + T_{G}}$

In the table of FIG. 25, a sub-frame length of 1.5 ms T_(Sub-Frame)=1500μs is used as an example to demonstrate how the number of symbols in thesub-frame is calculated. Since Δf is fixed at 12.5 kHz,

${T_{u} = {\frac{1}{\Delta \; f} = {80\mspace{14mu} {µs}}}},$

we can pick a Normal CP T_(G)=10μ for this sub-frame. We can also assumeRTG_(Sub-Frame)=0, and TTG_(Sub-Frame)>10μ to accommodate additionalpropagation delays to derive the following relationship:

$N_{SYM} = {{\frac{T_{{Sub}\text{-}{frame}} - {T\; T\; G_{{Sub}\text{-}{frame}}} - {R\; T\; {G\;}_{{Sub}\text{-}{frame}}}}{\frac{1}{\Delta \; f} + T_{G}} < \frac{1500 - 10 - 0}{80 + 10}} = 16.56}$

In one embodiment, the number of symbols in the sub-frame is 16, asshown in the table of FIG. 25.

From this example, regardless of the size of the transmission bandwidthto 5 MHz to 20 MHz, the number of symbols in a particular sub-frame isthe same when a particular CP length is chosen. For a particulardeployment channel condition, a particular CP is chosen; the RFperformance related to mobility for different transmission bandwidthremains roughly the same with the same RF overhead. The subscriber willenjoy the similar user experience in the system.

Additionally, as shown in the table of FIG. 25, the shorter the cyclicprefix the higher number of symbols can be fitted into a particularsub-frame, thereby providing higher throughput for the sub-frame. For aPico or Femto cell deployment, the delay spreads and round trip delaysare often small, so the base station and the associated mobile stationscan be configured to transmit with short cyclic prefixes (Short CP) toimprove frequency efficiency. For a Macro cell deployment, the coverageis often the important limitation. Due to high transmit RF power, Macrocell naturally have large cell site, which has increased the delayspreads and round trip delays for most radio signals. We can configurethe base station and mobile stations and the associated mobile stationswith long cyclic prefixes (Long CP) to combat rich multipath and largedelay spread so as to reduce inter-symbol interference (ISI). The FemtoCells, Pico Cells, and Macro Cells can be deployed simultaneously andeach can have optimized CP selections and frequency efficiency.

As discussed in relation to FIG. 12, the TTG(DL), TTG(UL), and RTG mayvary based upon to the size of the sub-frame and the correspondingcyclic prefix. The processor modules 616/622 may be suitably configuredto compute TTG (DL), TTG(UL) and the RTG values as follows:

The TTG (DL) may be calculated by using the following relationship:

-   -   TTG(DL)=(DL sub-frame duration) - (num of symbols in the DL        sub-frame)*(OFDM/OFDMA symbol duration (T_(SYM))), where    -   OFDM/OFDMA symbol duration=(cyclic prefix duration        (T_(G)))+(IFFT time (T_(u)))+(Windowing time (T_(WIN)))

Similarly, the TTG (UL) may be calculated by using the followingrelationship:

-   -   TTG(UL)=(UL sub-frame duration)−(num of symbols in the UL        sub-frame)*(OFDM/OFDMA symbol duration (T_(SYM))), where T_(SYM)        is calculated as shown above.

RTG is usually small and may be obtained by switching the time periodfrom transmit to receive mode. If RTG is required in the system design,RTG should also be defined in a time unit which can be divided evenly byall sampling times. Using the table in FIG. 14 as an example, theminimum time unit is T_(S)=3.125/5=0.625 μs. RTG for Normal CP sub-frameis 1.25 μs in this particular example. Alternatively, RTG can also beset to zero for the sub-frame.

As shown in FIG. 14, the TTG(DL), TTG(UL), and RTG, for a Long sub-frameand a Short cyclic prefix (CP), may be 3.75 μs, 2.5 μs, and 1.25 μsrespectively. Similarly, the TTG(DL), TTG(UL), and RTG, for the Longsub-frame and a Normal cyclic prefix, may be 60 μs, 58.75 μs, and 1.25μs respectively, and so on.

OFDM/OFDMA parameters in FIGS. 15-26 may share same OFDM/OFDMAparameters definition and functionality as FIG. 14, therefore thesedefinitions and the functionalities are not redundantly explainedherein.

FIG. 15 illustrates an exemplary table of basic OFDMA parameters for a3.5 bandwidth series (channel transmission bandwidths 3.25, 7, 14, 28,56, and 112 MHz) according to an embodiment of the invention. The FFTsize N_(FFT) is scalable from 512 to 16384. Similar to the 1.25bandwidth series explained above, an OFDM/OFDMA system with a fixedsubcarrier spacing value Δf=12.5 kHz may be used for the 3.5 bandwidthseries. As shown in FIG. 15, the sampling frequencies can be 6.4, 12.8,25.6, 51.2, 102.4, and 204.8 MHz. For this example, the number ofoccupied subcarriers for channel transmission bandwidth BW of 3.25, 7,14, 28, 56, and 112 MHz are 281, 561, 201, 1121, 2241, 4481 and 8961respectively.

Variable cyclic prefix durations plus a windowing time (e.g.,T_(G)+T_(WIN)≈2.97/10/16.72 μs) can be chosen based on the same Δf≈12.5kHz condition. The variable cyclic prefix durations are referred to asShort, Normal, and Long cyclic prefix respectively, and can be used forsmall, regular, and large cell site (FIG. 1) deployment. Differentcyclic prefixes may be selected for OFDM/OFDMA symbols in a sub-frame(e.g., short, large, and long). Different sub-frames (e.g., Short,Regular, Long) durations T_(Sub-frame) can be designed based ondifferent cyclic prefix durations such as Short, Normal, and Long cyclicprefix durations respectively. For example, for T_(Sub-frame)=0.5, 1,and 1.5 ms (FIG. 8), MHz, the T_(G)+T_(WIN) duration (overhead) for a3.5 MHz channel transmission bandwidth may be about 2.96/10/16.875 μsrespectively.

As mentioned above, according to an embodiment of the invention, theTTG(DL), TTG(UL), and RTG may vary based upon to the size of thesub-frame and the corresponding cyclic prefixes. For example, as shownin FIG. 15, the cyclic prefix duration of the TTG(DL), TTG(UL), and RTG,for a Long sub-frame and a Short cyclic prefix, may be about 6.56, 6.25,and 0.31 μs respectively. Similarly, the cyclic prefix duration of theTTG(DL), TTG(UL), and RTG, for the Long sub-frame and a Normal cyclicprefix, may be about 60 μs, 59.68 μs, and 0.31 μs respectively, and soon.

FIG. 16 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 1.25 MHz bandwidth series showing additional optimized overhead(T_(G)+T_(WIN)) values that may be used according to embodiments of theinvention. For example, overhead values for a 1.25 MHz channeltransmission bandwidth BW may be about 2.5/9.3716.87 for a Short cyclicprefix, Normal cyclic prefix, and Long cyclic prefix respectively(compared to 3.125/10/16.875 μs in FIG. 14), and so on.

FIG. 17 illustrates an exemplary table of basic OFDMA parameters for a3.5 bandwidth series showing additional overhead values that may be usedaccording to embodiments of the invention. For example, T_(G)+T_(WIN)durations that are similar to the 1.25 MHz bandwidth series explainedabove (FIG. 16), but are used for a 3.5 MHz bandwidth series.

FIGS. 18-23 are extensions of FIG. 16 showing values for the TTG (DL),TTG(UL) and RTG for 0.5, 0.675, 1, 1.25, 2, and 2.5 sub-frames accordingto various embodiments of the invention.

FIG. 18 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 1.25 MHz bandwidth series with a 0.5 ms sub-frame according to anembodiment of the invention. As shown in FIG. 18, the duration for theTTG(DL), TTG(UL), and RTG for a 0.5 ms sub-frame and a Short cyclicprefix (2.5 μs), may be 5, 2.5, and 2.5 μs respectively. Similarly, theduration for the TTG(DL), TTG(UL), and RTG for a 0.5 ms sub-frame and aLong cyclic prefix (15 μs), may be about 112.5, 110, and 2.5 μsrespectively, and so on.

FIG. 19 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 1.25 MHz bandwidth series with a 0.675 ms sub-frame according toan embodiment of the invention. For example, as shown in FIG. 19, theduration for the TTG(DL), TTG(UL), and RTG for a 0.675 ms sub-frame anda Short cyclic prefix (2.5 μs), may be 15, 12.5, and 2.5 μsrespectively. Similarly, the duration for the TTG(DL), TTG(UL), and RTGfor a 0.675 ms sub-frame and a Long cyclic prefix (15 μs), may be 93.75,91.25, and 2.5 μs respectively, and so on.

FIG. 20 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 1.25 MHz bandwidth series with a 1 ms sub-frame according to anembodiment of the invention. For example, as shown in FIG. 20, theduration for the TTG(DL), TTG(UL), and RTG for a 1 ms sub-frame with aShort cyclic prefix (2.5 μs), may be 10, 7.5, and 2.5 μs respectively.Similarly, the duration for the TTG(DL), TTG(UL), and RTG for a 0.675 mssub-frame and a Long cyclic prefix (15 μs), may be 31.25, 28,75, and 2.5μs respectively, and so on.

FIG. 21 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 1.25 MHz bandwidth series with a 1.5 ms sub frame according to anembodiment of the invention. For example, as shown in FIG. 21, theTTG(DL), TTG(UL), and RTG for a 1.5 ms sub-frame with a Short cyclicprefix (2.5 μs), may be 15, 12.5, and 2.5 μs respectively. Similarly,the duration for the TTG(DL), TTG(UL), and RTG for a 1.5 ms sub-frameand a Long cyclic prefix (15 μs), may be 46.875, 44.375, and 2.5 μsrespectively, and so on.

FIG. 22 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 1.25 MHz bandwidth series with a 2 ms sub frame according to anembodiment of the invention. For example, as shown in FIG. 22, theduration of the TTG(DL), TTG(UL), and RTG for a 2 ms sub-frame with aShort cyclic prefix (2.5 μs), may be 20, 17.5, and 2.5 μs respectively.Similarly, the duration for the TTG(DL), TTG(UL), and RTG for a 2 mssub-frame and a Long cyclic prefix (15 μs), may be about 62.5, 60, and2.5 μs respectively, and so on.

FIG. 23 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 5 MHz bandwidth series with a 2.5 ms sub-frame for transmission ina channel with a channel transmission bandwidth BW of 20 MHz accordingto an embodiment of the invention. For example, as shown in FIG. 23, theduration of the TTG(DL), TTG(UL), and RTG for a 2.5 ms sub-frame with aShort cyclic prefix (2.5 μs), may be 25, 22.5, and 2.5 μs respectively.Similarly, the duration for the TTG(DL), TTG(UL), and RTG for a 2.5 mssub-frame and a Long cyclic prefix (15 μs), may be about 78.125,75.625,and 2.5 μs respectively, and so on.

Alternatively, systems 600 and 700 may operate with different fixedsubcarrier spacing and hence provide different scalability properties.In this manner embodiments of the invention can offer compatibility withvarious communication systems. For example, FIGS. 24-26 illustrateexemplary tables of basic OFDM/OFDMA parameters for a 5 MHz bandwidthseries (channel transmission bandwidths 5, 7, 8.75, 10, 14, and 20 MHz)for a sub-carrier spacing Δf=10.9375 KHz, 12.5 KHz, and 25 KHz. Asub-carrier spacing of Δf=10.9375 KHz corresponds to that used in IEEE802.16e (WiMAX).

FIG. 24 illustrates an exemplary table of basic OFDMA parameters for aMHz bandwidth series for a sub-carrier spacing Δf=10.9375 which can notdivide all the RF bandwidths evenly, therefore it is not a good choiceaccording to embodiments of the invention. However, IEEE 802.16e versionof Mobile WiMAX has chosen Δf=10.9375 kHz for 5 MHz and 10 MHz channelbandwidths deployment. For some backward compatibility consideration,Δf=10.9375 or Δf=21.875 kHz can be used for other channel bandwidthsdeployment for IEEE 802.16m version of future WiMAX. According to thisembodiment of the invention, the FFT size N_(FFT) is scalable from 512to 2048. The sampling frequency (e.g., F_(S)=5.6, 11.2, 11.2, 11.2,22.4, and 22.4 MHz) is calculated for the 5, 7, 8.75, 10, 14, and 20 MHzchannel transmission bandwidths respectively as explained above. Thenumber of occupied subcarriers for channel transmission bandwidths of 5,7, 8.75, 10, 14, and 20 MHz may be 421, 589, 735, 841, 1177, 1681respectively in this example. A Short cyclic prefix, a Normal cyclicprefix, a Long cyclic prefix, and another Long cyclic prefix (CP2)durations of 2.857, 11.428, 17.142, and 22.857 μs can be chosen for the5 MHz bandwidth series.

Different sub-frames durations T_(Sub-frame) can be designed based ondifferent cyclic prefix durations such as Short, Long, and Normal cyclicprefix durations. For example, sub-frame durations may beT_(Sub-frame)=0.5, 0.675, 1, 1.5, 2, and 2.5, and the cyclic prefixduration for these sub-frames may be selected from the above cyclicprefix values. For example, for a 0.5 ms sub-frame duration at 5 MHzchannel transmission bandwidth BW, a Short cyclic prefix of 2.857 μs canbe selected thereby allowing 5 OFDM/OFDMA symbol per frame to betransmitted. The duration for the TTG(DL), TTG(UL), and RTG (FIG. 12)may vary based upon to the size of the sub-frames and the correspondingcyclic prefixes. For example, as shown in FIG. 25, for the 5 MHzbandwidth series, the duration for the TTG(DL) or TTG(UL), for a 0.5 mssub-frame and a Short cyclic prefix (2.857 μs), may be 28.571 μs.Similarly, the duration for the TTG(DL) or TTG(UL), for a 0.5 mssub-frame and a Long cyclic prefix (17.142 μs), may be 65.71 μs, and soon.

FIG. 25 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 5 MHz bandwidth series according to an embodiment of theinvention. In this example, the FFT size N_(FFT) is scalable from 512 to2048. A fixed subcarrier spacing value Δf=12.5 kHz (similar to the 1.25bandwidth series) may be used for the 5 MHz bandwidth series.

The sampling frequency (e.g., F_(S)=6.4, 12.8, 12.8, 12.8, 25.6, and25.6 MHz) is calculated for the 5, 7, 8.75, 10, 14, and 20 MHz channeltransmission bandwidth BW respectively as explained above. For thisexample, the number of occupied subcarriers for channel transmissionbandwidth BW of 5, 7, 8.75, 10, 14, and 20 MHz is 401, 561, 701, 801,1121, 4481 and 1601 respectively. A Short cyclic prefix, a Normal cyclicprefix, a Long cyclic prefix, and another Long cyclic prefix (CP2)durations of 2.5, 10, 15, and 20 μs can be chosen for these channeltransmission bandwidths.

Different sub-frames durations T_(Sub-frame) can be designed based ondifferent cyclic prefix durations such as Short, Long, and Normal cyclicprefix durations. For example, T_(Sub-frame)=0.5, 0.675, 1, 1.5, 2, and2.5, and the cyclic prefix duration for these sub-frames may be selectedfrom 2.5, 10, 15, and 20 μs cyclic prefix duration T_(G) values. Forexample, for a 0.5 ms sub-frame duration at a channel transmissionbandwidth of 5 MHz, a cyclic prefix duration of 2.5 μs can be selected.As mentioned above, the TTG(DL), TTG(UL), and RTG (FIG. 12) may varybased upon to the size of the sub-frame and the corresponding cyclicprefixes. For example, as shown in FIG. 18, for the transmissionbandwidth BW of 5 MHz, the duration for the TTG(DL) or TTG(UL), for a0.5 ms sub-frame and a Short cyclic prefix (2.5 μs), may be 5 μs.Similarly, the duration for the TTG(DL) or TTG(UL), for a 0.5 mssub-frame and a Long cyclic prefix (15 μs), may be 120 μs, and so on.

FIG. 26 illustrates an exemplary table of basic OFDM/OFDMA parametersfor a 5 MHz bandwidth series with a subcarrier spacing Δf≈25 KHzaccording to an embodiment of the invention. The FFT size N_(FFT) isscalable from 256 to 1024 (e.g., 256, 512, 512, 512, 1024, and 1024).The sampling frequency (e.g., F_(S)=6.4, 12.8, 12.8, 12.8, 25.6, and25.6 MHz) is calculated for the 5, 7, 8.75, 10, 14, and 20 MHz channeltransmission bandwidths BW respectively as explained above. The numberof occupied subcarriers for these channel transmission bandwidths can be201, 281, 351, 401, 561, and 801 respectively. A Short cyclic prefix, aNormal cyclic prefix, a Long cyclic prefix, and another Long cyclicprefix (CP2) durations of 2.857 μs, 11.428 μs, 17.142 μs, and 22.857 μscan be chosen for these channel transmission bandwidths.

Different sub-frames durations T_(Sub-frame) can be designed based ondifferent cyclic prefix durations such as Short, Long, and Normal cyclicprefix durations. For example, sub-frame durations may beT_(Sub-frame)=0.5, 0.675, 1, 1.5, 2, and 2.5, and the cyclic prefixduration for these sub-frames may be selected from the above cyclicprefix values. For example, for a 0.5 ms sub-frame duration at 5 MHzbandwidth, a Short cyclic prefix of 2.5 μs can be selected therebyallowing 11 OFDM/OFDMA symbols per frame to be transmitted in thisframe. The duration for the TTG(DL), TTG(UL), and RTG (FIG. 12) may varybased upon to the size of the sub-frames and the corresponding cyclicprefixes. For example, as shown in FIG. 26, for a channel transmissionbandwidth of 5 MHz, the duration for the TTG(DL), or TTG(UL), for a 0.5ms sub-frame and a Short cyclic prefix (2.5 μs), may be 32.5 μs.Similarly, the duration for the TTG(DL), or TTG(UL), for a 0.5 mssub-frame and a Long cyclic prefix (15 μs), may be 60 μs, and so on.

FIG. 27 illustrates a flowchart showing an OFDM/OFDMA process 2700 forcreating a frame structure with a variable cyclic prefix according toembodiments of the invention. The various tasks performed in connectionwith these processes may be performed by software, hardware, firmware, acomputer-readable medium having computer executable instructions forperforming the process method, or any combination thereof. It should beappreciated that process 2700 may include any number of additional oralternative tasks. The tasks shown in FIGS. 27 need not be performed inthe illustrated order, and these processes may be incorporated into amore comprehensive procedure or process having additional functionalitynot described in detail herein. For illustrative purposes, the followingdescription of process 2700 may refer to elements mentioned above inconnection with FIGS. 6-26. In various embodiments, portions of process2700 may be performed by different elements of systems 600-700 e.g.,transceivers and processors. OFDM/OFDMA process 2700 may share sameOFDM/OFDMA definitions and functionalities as explained above in thecontext of FIGS. 6-26, therefore these definitions and thefunctionalities are not redundantly explained herein.

Process 2700 may begin with the OFDM/OFDMA transmitter 701 receivingtime domain OFDM data symbols for transmission on an RF channel (task2702). Next, the cyclic prefix selector 709 selects a cyclic prefix froma plurality of variable length cyclic prefixes (task 2704). The cyclicprefix may be selected from a plurality of cyclic prefixes available forthe RF channel. For example, as shown in FIG. 14, the RF channel maycomprise a plurality of variable length cyclic prefixes that range from5-864 samples for various channel transmission bandwidths in the RFchannel.

As shown in FIG. 14, for a 1.25 MHz channel transmission bandwidth BW, aset of variable length cyclic prefix comprises a Short, a Normal and aLong cyclic prefix length comprising 5, 16, and 27 samples respectively.These cyclic prefixes may be scaled to obtain the set of cyclic prefixesfor each of the other channel transmission BW (RF channels). Forexample, a short cyclic prefix (e.g., 5 samples) can be scaled to obtaina 10, 20, 40 and 80 samples for the channel transmission bandwidths BWof 2.5, 5, 10, 20, and 40 MHz respectively, and so on. For these channeltransmission bandwidths, the cyclic prefix duration of 3.125 μs can thenbe calculated as explained above in the context of discussion of FIG.14.

Process 2700 then adds the selected cyclic prefix into each of the timedomain OFDM/OFDMA data symbols to obtain a plurality of OFDM frames(task 2706) using the add cyclic prefix module 710. The selected cyclicprefix may be in the form of digital samples of the corresponding cyclicprefix duration. Process 2700 may then transmits the OFDM frames on theradio channel such as the radio channel 714 (task 2708). In this manner,process 2700 adds the OFDM frames to a variable size sub-frame prior totransmitting the OFDM frames on the channel.

According to embodiments of the invention, these variable length cyclicprefixes can be used for small, regular, and large cell site deploymentas explained above to improve bandwidth efficiency (bit/Hz) of thesystem. Furthermore, selecting different cyclic prefixes for OFDM/OFDMAsymbols in a sub-frame for a base station allows for supportingdifferent types of base station cells and cell coverage areas. Thereby,network deployment may be simplified and made more flexible byeliminating the need for the entire network to select the same cyclicprefixes regardless of the different requirements on each base stationfor its cell coverage area.

While various embodiments of the invention have been described above, itshould be understood that they have been presented by way of exampleonly, and not by way of limitation. Likewise, the various diagrams maydepict an example architectural or other configuration for thedisclosure, which is done to aid in understanding the features andfunctionality that can be included in the disclosure. The disclosure isnot restricted to the illustrated example architectures orconfigurations, but can be implemented using a variety of alternativearchitectures and configurations. Additionally, although the disclosureis described above in terms of various exemplary embodiments andimplementations, it should be understood that the various features andfunctionality described in one or more of the individual embodiments arenot limited in their applicability to the particular embodiment withwhich they are described. They instead can, be applied, alone or in somecombination, to one or more of the other embodiments of the disclosure,whether or not such embodiments are described, and whether or not suchfeatures are presented as being a part of a described embodiment. Thusthe breadth and scope of the present disclosure should not be limited byany of the above-described exemplary embodiments.

In this document, the term “module” as used herein, refers to software,firmware, hardware, and any combination of these elements for performingthe associated functions described herein. Additionally, for purpose ofdiscussion, the various modules are described as discrete modules;however, as would be apparent to one of ordinary skill in the art, twoor more modules may be combined to form a single module that performsthe associated functions according embodiments of the invention.

In this document, the terms “computer program product”,“computer-readable medium”, and the like, may be used generally to referto media such as, memory storage devices, or storage unit. These, andother forms of computer-readable media, may be involved in storing oneor more instructions for use by processor to cause the processor toperform specified operations. Such instructions, generally referred toas “computer program code” (which may be grouped in the form of computerprograms or other groupings), when executed, enable the computingsystem.

It will be appreciated that, for clarity purposes, the above descriptionhas described embodiments of the invention with reference to differentfunctional units and processors. However, it will be apparent that anysuitable distribution of functionality between different functionalunits, processors or domains may be used without detracting from theinvention. For example, functionality illustrated to be performed byseparate processors or controllers may be performed by the sameprocessor or controller. Hence, references to specific functional unitsare only to be seen as references to suitable means for providing thedescribed functionality, rather than indicative of a strict logical orphysical structure or organization.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; and adjectivessuch as “conventional,” “traditional,” “normal,” “standard,” “known”,and terms of similar meaning, should not be construed as limiting theitem described to a given time period, or to an item available as of agiven time. But instead these terms should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable, known now, or at any time in the future. Likewise, a group ofitems linked with the conjunction “and” should not be read as requiringthat each and every one of those items be present in the grouping, butrather should be read as “and/or” unless expressly stated otherwise.Similarly, a group of items linked with the conjunction “or” should notbe read as requiring mutual exclusivity among that group, but rathershould also be read as “and/or” unless expressly stated otherwise.Furthermore, although items, elements or components of the disclosuremay be described or claimed in the singular, the plural is contemplatedto be within the scope thereof unless limitation to the singular isexplicitly stated. The presence of broadening words and phrases such as“one or more,” “at least,” “but not limited to”, or other like phrasesin some instances shall not be read to mean that the narrower case isintended or required in instances where such broadening phrases may beabsent.

Additionally, memory or other storage, as well as communicationcomponents, may be employed in embodiments of the invention. It will beappreciated that, for clarity purposes, the above description hasdescribed embodiments of the invention with reference to differentfunctional units and processors. However, it will be apparent that anysuitable distribution of functionality between different functionalunits, processing logic elements or domains may be used withoutdetracting from the invention. For example, functionality illustrated tobe performed by separate processing logic elements, or controllers, maybe performed by the same processing logic element, or controller. Hence,references to specific functional units are only to be seen asreferences to suitable means for providing the described functionality,rather than indicative of a strict logical or physical structure ororganization.

Furthermore, although individually listed, a plurality of means,elements or method steps may be implemented by, for example, a singleunit or processing logic element. Additionally, although individualfeatures may be included in different claims, these may possibly beadvantageously combined. The inclusion in different claims does notimply that a combination of features is not feasible and/oradvantageous. Also, the inclusion of a feature in one category of claimsdoes not imply a limitation to this category, but rather the feature maybe equally applicable to other claim categories, as appropriate.

1. An OFDM communication system comprising: a plurality of radiofrequency (RF) channels, wherein the RF channels comprise dissimilarbandwidths; and a transmitter for providing a plurality of OFDMsubcarriers, wherein the OFDM subcarriers comprise a fixed subcarrierspacing chosen such that the OFDM subcarriers are scalable in number toutilize any of the RF channels.
 2. The OFDM communication system ofclaim 1, wherein the fixed subcarrier spacing that can divide theallocated bandwidth evenly is chosen to optimize bandwidth efficiency.3. The OFDM communication system of claim 1, wherein the fixedsubcarrier spacing that can divide a channel raster of 200 KHz or 250KHz evenly is chosen to optimize bandwidth efficiency.
 4. The OFDMcommunication system of claim 1, wherein the fixed subcarrier spacingcomprises one of the group consisting of about 10 kHz, 12.5 KHz, andmultiple of 10 kHz or 12.5 kHz, such as 20 kHz, and 25 KHz.
 5. The OFDMcommunication system of claim 1, wherein the fixed subcarrier spacingcomprises 10.9375 kHz or 21.875 kHz.
 6. The OFDM communication system ofclaim 1, wherein the OFDM communication system is an OFDMA communicationsystem.
 7. An OFDM communication system comprising a plurality of radiofrequency (RF) channels, wherein the RF channels comprise dissimilarbandwidths, the OFDM communication system comprising: a transmitter forproviding a plurality of OFDM subcarriers, wherein the OFDM subcarrierscomprise a fixed subcarrier spacing chosen such that the OFDMsubcarriers are scalable in number to utilize any of the RF channels;and a processor module operable to provide a flexible radio framestructure comprising a plurality of variable length cyclic prefixesoperable for the RF channels.
 8. The OFDM communication system of claim7, wherein the variable length cyclic prefixes comprise a set ofpredefined cyclic prefix durations for each RF channel.
 9. The OFDMcommunication system of claim 8, wherein the same set of cyclic prefixdurations is used by multiple or all RF channels.
 10. The OFDMcommunication system of claim 7, wherein each of the variable lengthcyclic prefixes can be dynamically selected for each of the RF channels.11. The OFDM communication system of claim 7, further comprising aplurality of OFDM frames each comprising one of the variable lengthcyclic prefixes, and one of a plurality of data symbols.
 12. The OFDMcommunication system of claim 11, further comprising a plurality ofvariable size sub-frames each comprising a subset of the OFDM frames.13. The OFDM communication system of claim 12, further comprising aplurality of timing gaps associated with at least one of the variablesize sub-frames, wherein the timing gaps are calculated based, at leastin part, on a duration of at least one of the variable length cyclicprefixes.
 14. The OFDM communication system of claim 13, wherein thetiming gaps provide protection for timing variations at signalreception.
 15. The OFDM communication system of claim 14, wherein thetiming gap is placed at the end of a downlink sub-frame.
 16. The OFDMcommunication system of claim 14, wherein the timing gap is placed atthe beginning of an uplink sub-frame.
 17. The OFDM communication systemof claims 15 and 16, wherein the transmit-transition gap (TTG) of aframe is consisted of the timing gap from downlink sub-frame (TTG-DL)plus the timing gap from uplink sub-frame (TTG-UL).
 18. The OFDMcommunication system of claim 12, further comprising a plurality ofvariable radio frame configurations for transmission through one of theRF channels, wherein the variable radio frame configurations comprise asubset of the variable size sub-frames.
 19. The OFDM communicationsystem of claim 7, wherein the set of cyclic prefix durations are chosenbased at least in part on a condition of each of the RF channels.
 20. Acommunication system comprising: at least one base station supportingvariable cyclic prefix durations, wherein the variable cyclic prefixdurations are chosen based on a cell coverage area of the at least onebase station; and a processor module for providing a flexible radioframe structure utilizing the variable size cyclic prefix durations,wherein the flexible radio frame structure is used by the at least onebase station for transmitting data wirelessly to a mobile station. 21.The communication system of claim 20, wherein the variable cyclic prefixdurations are chosen based on a fixed subcarrier spacing.
 22. Thecommunication system of claim 21, wherein the fixed subcarrier spacingcomprises 12.5 KHz.
 23. The communication system of claim 22, whereinthe variable cyclic prefix durations, plus a timing window, comprise oneof the group consisting of: about 3.125 μs, 10 μs, and 16.875 μs for an×1.25 MHz bandwidth series.
 24. The communication system of claim 22,wherein the variable cyclic prefix durations, plus a timing window,comprise one of the group consisting of: about 3.281 μs, 10 μs, and16.178 μs for a n×3.5 MHz bandwidth series.
 25. The communication systemof claim 22, wherein the variable cyclic prefix durations, plus a timingwindow, comprise one of the group consisting of: about 2.5 μs, 9.375 μs,and 16.875 μs for a n×1.25 MHz and a n×3.25 MHz bandwidths series. 26.The communication system of claim 22, wherein the variable cyclic prefixdurations comprise one of the group consisting of: about 2.5 μs, 10 μs,15 μs, and 20 μs for a 5 MHz bandwidth series.
 27. The communicationsystem of claim 21, wherein the fixed subcarrier spacing comprises about10.9375 KHz.
 28. The communication system of claim 27, wherein thevariable cyclic prefix durations comprise one of the group consistingof: about 2.857 μs, 11.428 μs, 17.142 μs, and 22.857 μs for a 5 MHzbandwidth series.
 29. The communication system of claim 21, wherein thefixed subcarrier spacing comprises about 25 KHz.
 30. The communicationsystem of claim 29, wherein the variable cyclic prefix durationscomprise one of the group consisting of: about 2.5 μs, 10 μs, 15 μs, and20 for a 5 KHz bandwidth series.
 31. The communication system of claim20, wherein a first cyclic prefix duration for a first base stationdiffers from a second cyclic prefix duration for a second base station.32. The communication system of claim 20, wherein the flexible radioframe structure comprises a 10 ms radio frame.
 33. The communicationsystem of claim 20, wherein the flexible radio frame structure comprisessub-frames which comprise sub-frame duration options based on thevariable cyclic prefix durations.
 34. The communication system of claim33, wherein the sub-frame duration options comprise one of the groupconsisting of: about 0.5 ms, 0.675 ms, 1 ms, 1.25 ms, 1.5 ms, 2 ms, and2.5 ms.
 35. The communication system of claim 33, wherein the sub-frameduration options allow a system to reduce interference with systemsbased on a plurality of industry standards.
 36. The communication systemof claim 35, wherein the industry standards comprise one of the groupconsisting of: Third Generation Partnership Project Long Term Evolution(3GPP LTE), Third Generation Partnership Project 2 Ultra MobileBroadband (3Gpp2 UMB), Time Division-Synchronous Code Division MultipleAccess (TD-SCDMA), and Wireless Interoperability for Microwave Access(WiMAX).
 37. The communication system of claim 33, wherein thesub-frames comprise a plurality of sub-frame sizes.
 38. Thecommunication system of claim 37, wherein the variable cyclic prefixdurations are selected for each of the sub-frame sizes.
 39. Thecommunication system of claim 33, wherein the processor module isfurther operable to calculate a plurality of timing gaps associated withat least one of the sub-frames, wherein the timing gaps are calculatedbased in part on the variable cyclic prefix durations.
 40. Thecommunication system of claim 39, wherein the timing gaps comprise oneof the group consisting of: a transmission timing gap for an uplinkTTG(UL), a transmission timing gap for a downlink uplink TTG(DL), and atransmit-receive timing gap (RTG).
 41. An OFDM/OFDMA radio framestructure for communication in an RF channel in a wireless network, theradio frame structure comprising: a plurality of OFDM symbols eachcomprising a variable cyclic prefix duration and an at least one OFDMdata symbol; a plurality of variable size sub-frames formed from asubset of the OFDM symbols; a plurality of radio frames for transmittinga subset of the variable size sub-frames through the RF channel; and aplurality of timing gaps associated with the radio frames for providinga protection for timing variations at signal reception, wherein thetiming gaps are calculated based at least in part on the variable cyclicprefix duration.
 42. The OFDM/OFDMA radio frame structure of claim 41,further comprising a sub-frame structure for the variable sizesub-frames comprising: a downlink sub-frame; and an uplink sub-frame,wherein the uplink sub-frame and the downlink sub-frame use the same RFchannel for communication, and wherein the downlink sub-frame operateswith the uplink sub-frame to achieve a maximum transmit-receivetransition time gap.
 43. The OFDM/OFDMA radio frame structure of claim42, wherein the transmit-receive transition time gap is larger than around-trip delay from a mobile station at the edge of a communicationcell of a base station to the base station.
 44. The OFDM/OFDMA radioframe structure of claim 41, wherein the variable cyclic prefix durationis based on a number of OFDM subcarriers used on the RF channel.
 45. TheOFDM/OFDMA radio frame structure of claim 41, wherein the networkcomprises an IEEE 802.16m standard network.
 46. A communication systemcomprising a plurality of RF channels, wherein the RF channels comprisedissimilar channel bandwidths, the communication system comprising: aninverse fast Fourier transform module operable for transforming aplurality of frequency domain data symbols into a plurality of timedomain data symbols respectively; a cyclic prefix selector moduleoperable for selecting a cyclic prefix from a plurality of variablelength cyclic prefixes to obtain a selected cyclic prefix; and an addcyclic prefix module operable for adding the selected cyclic prefix intoeach of the time domain data symbols to obtain a plurality of OFDMframes.
 47. The communication system of the claim 46, further comprisinga processor module operable for: providing a plurality of variable sizesub-frames formed from a subset of the OFDM frames; providing aplurality of radio frames for transmitting a subset of the variable sizesub-frames through at least one of the RF channels; and calculating aplurality of timing gaps associated with at least one of the variablesize sub-frames for providing a protection for timing variations atsignal reception, wherein the timing gaps are calculated based at leastin part on a cyclic prefix duration of the selected cyclic prefix. 48.The communication system of the claim 47, wherein a cyclic prefixduration of the selected cyclic prefix is determined based on a samplingrate of one of the RF channels.
 49. A method for communication in acommunication system, the method comprising: receiving a plurality oftime domain data symbols for transmission on a radio channel; selectinga cyclic prefix from a plurality of variable length cyclic prefixes toobtain a selected cyclic prefix; and adding the selected cyclic prefixinto each of the time domain data symbols to obtain a plurality of OFDMframes.
 50. The method of claim 49, further comprising transmitting theOFDM frames on the radio channel.
 51. The method of claim 50, furthercomprising adding the OFDM frames to a flexible sub-frame prior totransmitting.
 52. The method of claim 51, wherein the flexible sub-framecomprises a timing gap for providing a protection for timing variationsat signal reception, wherein the timing gap is calculated, based atleast in part, on a cyclic prefix duration of the selected cyclicprefix.
 53. The method of claim 51, further comprising forming a radioframe comprising the flexible sub-frame prior to transmitting.
 54. Acomputer-readable medium for a communication system, comprising programcode for: receiving a plurality of time domain data symbols fortransmission on a radio channel; selecting a cyclic prefix from aplurality of variable length cyclic prefixes for the radio channel toobtain a selected cyclic prefix; and adding the selected cyclic prefixinto each of the time domain data symbols to obtain a plurality of OFDMframes.
 55. The computer-readable medium of claim 54, further comprisingprogram code for adding the OFDM frames to a flexible sub-frame prior totransmitting the OFDM frames on the radio channel.
 56. Thecomputer-readable medium of claim 55, further comprising program codefor: providing a plurality of variable size sub-frames formed from asubset of the OFDM frames; providing a plurality of radio frames fortransmitting a subset of the variable size sub-frames through the radiochannel; and calculating a plurality of timing gaps associated with atleast one of the variable size sub-frames for providing a protection fortiming variations at signal reception, wherein the timing gaps arecalculated based at least in part on a cyclic prefix duration of theselected cyclic prefix.
 57. The computer-readable medium of claim 55,further comprising program code for forming a radio frame comprising theflexible sub-frame prior to transmitting.